This invention relates generally to machine power feed mechanisms and, more specifically, to an improved clutch for a quill power feed mechanism of a milling machine which remains engaged throughout the course of drilling or boring operations, but simultaneously prevents undue strain and stress from being imposed on the mechanism.
Designers of machine tools must be concerned with the effects of over-stressing machine component parts. Numerous machines, particularly machine tools, are fitted with trip devices to prevent such undue strain and stress being imposed on a part. In milling machines, trip devices are normally present in the quill feed mechanism so that if the milling cutter or the quill meets any unyielding obstruction, either in its forward or reverse feed, the feed will be arrested until the operator removes the obstruction.
Such trip mechanisms are usually factory installed and designed to disengage the power feed when resistance to the feed exceeds a pre-set value. In most standard milling machines, the quill feed mechanism is provided an overload clutch set at the factory to disengage if the down pressure on the quill exceeds 200 pounds.
Although these factory-set trip devices have served their intended purpose of preventing feed mechanism damage, they have been far from ideal. Often the factory-set disengage pressure of 200 pounds restricts the size of drills which can be used with the power feed. Further, in many cases the 200 pound trip value is excessive for the job being done, and a much lower value would be preferable. Moreover, in precision drilling or boring operations where a micrometer quill stop is utilized to limit the downward travel of the quill, the prior trip devices typically disengage the power feed drive mechanism several thousandths of an inch above the desired depth, requiring manual feed for completion. This disengagement of the quill power feed mechanism, which also occurs each time the quill is raised, often delays the milling operation and increases costs.
Accordingly, there has been a need for an improved clutch for a quill power feed mechanism which enables an operator to adjust the amount of downward quill pressure to the needs of the operation, thereby substantially decreasing the likelihood of machine damage in the case of a miscue. Additionally, there exists a need for a clutch which enables the power mechanism to feed the quill to a positive stop at the exact depth desired when using a micrometer quill stop in precision operations. Further, an improved clutch for a quill power feed mechanism is needed which, in addition to those features noted above, is sturdy, dependable and capable of maintaining the same quill pressure during successive operations to insure the repeatability of depth accuracy. Finally, there exists a significant need for an economical clutch which can be easily installed in standard milling machines, such as Bridgeport-type machines, which slips at the pre-set torque value rather than disengaging, to eliminate the disengage and engage time which slows milling operations. The present invention fulfills these needs and provides other related advantages.